Many workpieces are designed to operate in a dynamically balanced rotational condition carrying attached bodies or tools having a specific mass. Therefore, when the workpiece is disattached from the bodies or tools it will have a certain inherent rotational unbalance. The present invention provides an improved method for offsetting this inherent rotational unbalance of a workpiece in order to accurately complete a dynamic balancing study of the same workpiece. Such balancing studies are in order, for instance, when the workpiece is first manufactured or when it is being altered for some after-market design. These offsetting methods during dynamic balancing studies are traditionally called "bobweighting" and have specifically been applied to the balancing of workpieces such as flywheels and crankshafts which have a plurality of offset throws. For the purposes of illustration and simplicity the method of the present invention will be described as applied to crankshaft balancing. The method, however, is applicable to all balancing operations which require some form of bobweighting and the present description is not intended to be limited solely to use with crankshafts.
A crankshaft is designed to operate a rotationally balanced condition while carrying a plurality of pistons, each having a specific mass. Any attempt to dynamically balance the individual crankshaft must account for the unbalance created by the missing mass of each piston which would normally be fixed to the various throws. A traditional method of providing the necessary duplication of mass is to counterweight or bobweight the workpiece or crankshaft. The bobweights must be accurately constructed and individually attached to each throw of the crankshaft to ensure the accurate duplication of running conditions. While this methodology is proven to be acceptable and accurate in duplicating crankshaft running conditions, the use of such bobweights creates an undesirable complexity in the balancing operation due to the manpower and time consumption needed to properly attach and remove each physical bobweight.
Recent attempts to improve upon the traditional method of bobweighting have focused on electronically offsetting the measured balance reading received from the rotating workpiece. These attempts to electronically offset the bobweight unbalance usually rely either upon a measurement of unbalance from a master part previously balanced with bobweights or master data relating to proper bobweighting provided by the manufacturer. The value of the bobweight unbalance measurement is then used to reduce subsequent unbalance readings taken on subsequent similar parts. Problems have been encountered, however, with such an electronic unbalance offset due to inefficiencies in the electrical circuitry of the components used to analyze the electronic signals and modify the electronic readings of unbalance. It has been found that there is a drift error in the amount and phase angle of the unbalance vector signals from cold startup of the electronic components until the temperature of the components has stablized. this drift error usually amounts to a 2 to 3% change in the amount and phase of the unbalance vector. A 2 to 3% drift error is completely undesirable for a situation where it is necessary to balance a workpiece to fine tolerances. For instance, if it is desired to balance a workpiece to 15 ounce inches of unbalance with a tolerance of plus or minus 0.25, the 2 to 3% drift in the electronics is the equivalent of the tolerance for the balance.
Therefore, a need remains for an improved bobweight system which eliminates the need for use of physical bobweights and yet overcomes the problems encountered by the current electronic bobweight unbalance elimination systems.